I've had that sound at that exact RPM range on both of my SVXs. Both were FWD. The first one had the original tranny, and even at 98k miles, there was no slip in shifting. My current one has 135k miles, a rebuilt tranny with 20k on it, and it also makes the noise (but less often than the last one). Has no slip in shifting, but there is some serious weight movement when it shifts from 1 - 2 between 1.5k and 2.5k RPM.

Neither tranny seemed to ever get worse in the noise or in shifting. I still don't know what causes it, but maybe that'll make you feel a bit better.